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Simple tension

  1. Jan 6, 2008 #1
    I know this may sound very simple, but I am actually a bit confuses about it.

    Say, in a pendulum swinging in a vertical plane. How does one calculate the tension in the string ( no mass) ,which is connected to a bob (mass m), at various points, say like and angle beta with the vertical?

    I know its a very simple idea, but it is actually confusing me. I think I am doing it wrong. If someone could run thru it quickly, I mite see my mistake.

    Cheers guys :redface:
     
  2. jcsd
  3. Jan 6, 2008 #2
    Draw a free body diagram of the bob. What forces are acting on it? The sum of all forces in X direction equals zero and like wise in Y direction. Once you derive the magnitude of the tension in X and Y direction, how would you calculate the magnitude of that vector?
     
  4. Jan 6, 2008 #3
    Would the sum in x direction be zero since the pendulum is swinging? Isn't it acceleration in a horizontal direction?
     
  5. Jan 6, 2008 #4

    Doc Al

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    Haven't we discussed this at length in your other thread? :grumpy:

    I told you several times what to do. Did you try it?

    (Also: Don't post the same question in multiple threads!)
     
  6. Jan 6, 2008 #5
    This is actually a different question :redface:

    It's a bit different but no matter how I try it my answer is wrong. I must be seriously flawed somewhere
     
  7. Jan 6, 2008 #6

    Doc Al

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    I would not conclude that you were seriously flawed, but your approach may well be. :smile:

    As always, identify the forces and apply Newton's 2nd law. Hint: The acceleration has a radial and a tangential component. Treat them separately.
     
  8. Jan 6, 2008 #7
    Thanks, I'll try that in a second. But just to give you an idea of my reasoning, say the pendulum passes through the vertical.

    My diagram says there are 2 forces on the bob, weight acting down, and tension in string acting upward. And these 2 should be equal in magnitude. Is THIS right?
     
  9. Jan 6, 2008 #8

    Doc Al

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    This is right.
    This is wrong.

    Hint: What's the acceleration? Hint 2: What kind of motion is this?
     
  10. Jan 6, 2008 #9
    Circular?

    So my equation for the sum of y forces would be:

    T - mg = (m(v)^2)/R ??
     
  11. Jan 6, 2008 #10

    Doc Al

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    Right!
     
  12. Jan 6, 2008 #11
    Hoorah! :rolleyes:

    Ok so that's for the vertical position, and it seems straight forward, I just forgot about the acceleration. But what about say at an angle theta with the vertical?
     
  13. Jan 6, 2008 #12

    Doc Al

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    Analyze force components parallel to the string. Apply Newton's 2nd law. (Sound familiar?) Hint: Find the speed.
     
  14. Jan 6, 2008 #13
    But if that angle is its maximum, wont speed be zero?
     
  15. Jan 6, 2008 #14

    Doc Al

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    Sure, if the angle is the maximum angle. (But you just said angle theta. :wink:) I trust you can solve your earlier problem now?
     
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